1. Field of Invention
The present invention relates to hardware designed for electromagnetic compatibility (EMC) immunity and, more specifically, to a standard Universal Serial Bus (USB) interface having improved EMC.
2. Description of Prior Art
Universal Serial Bus, or USB, architecture is becoming a popular substitute for parallel or serial bus architecture, and USB ports, in addition to parallel and serial ports, are provided as standard equipment on most computers manufactured today. USB specifications are available at the URL http://www.usb.org.
USB requires a single pair of twisted wires in a cable for data transmission, while a second pair provides power from a host to a peripheral unit. Three signaling conditions are utilized. Data packets are sent using differential signaling followed by an end of packet, where both lines go low for two clocks. A third state is the idle state where neither line is driven, but one is pulled up by a resistor at the device end. The host controls the packet flow such that when the peripheral is sending data, the host is in receive mode, and vice-versa, and no collisions are possible.
While the USB interface has brought more flexibility, packetized data integrity, and automatic configuration to PC-connected peripherals, the interface defined by the USB-Integrator's s Forum (USB-IF) has inherent deficiencies in meeting today's European Norm (EN) EMC immunity requirements. Although the published USB standards (Rev. 1.1 or 2.0) require all devices bearing the USB logo to meet today's EN requirements for “CE” mark, very few commercially available USB peripherals actually do so on the immunity side. The forum for acceptance of USB devices, known as “Plugfest,” focuses primarily on the functional aspects of compliance to the USB standard in terms of link level protocols and signal integrity, neglecting the CE mark EMC requirements invoked in the USB standard.
Under the broad term EMC Immunity, as defined in EN55024, are specific tests such as ESD, EFT, RFI, and CRFI that apply and are required to meet CE mark. In particular peripheral markets, namely peripherals for use in a point of sale environment, such as POS-printers, the standard CE mark tests are required, but at higher voltage treatment levels. As such, EMC immunity can quickly become the major obstacle in the use of USB in a POS environment. Due to these higher level Immunity requirements, POS has been accurately depicted as an industrial level of Immunity.
In terms of ESD immunity, because the USB device controller is connected to the physical interface conductors and contains RAM registers that can be easily upset by large ESD-induced signal transients, immunity to ESD is inherently poor. In terms of EFT immunity, some of the same problems as with ESD are encountered, except that instead of altered logic states, either the Early End of Packet (Early EOP) or Babble are caused due to poor recovery mechanisms in the hardware.
The reason these conditions are reached during EFT testing is an unlucky choice of Start Of Frame (SOF) packet interval at an exact harmonic of the likewise arbitrary pulse frequency chosen by the EFT committee as the basis for the EFT test. Without improving basic signal robustness or cabling, a choice of slightly different SOF interval would eliminate the chance of having several SOF packets in a row lost to interference during EFT testing. In fact, a 1 ms SOF interval ensures that eventually, up to 15 SOF in a row will be lost to a 15 ms burst of 5 KHz energy injected on either the signal cable or AC mains to the POS-device (i.e., printer) during the EFT test, which will likely result in a disconnected device having no fully USB-compliant method of automatic reconnection.
While use of common mode chokes are known to control fault currents in lower speed electronic interfaces, their use is expressly not recommended by the USB-IF in USB interfaces, presumably due to the topology and signaling methods used in USB.
3. Objects and Advantages
It is therefore a principal object and advantage of the present invention to provide improved EMC immunity is a USB interface.
It is an additional object and advantage of the present invention to provide improved EMC immunity while not degrading signal integrity.
Other objects and advantages of the present invention will in part be obvious, and in part appear hereinafter.